Dreamweaver CS5 on Mac was fine for me...

In fact, the question is plural, do you use many of the features present, or proposed, by CS5... like, use of server behavior, widgets, some kind of table layout, Spry library... etc...

today all this has disappeared from Dreamweaver, and is no longer available on the web... this doesn't mean that sites developed in this way are no longer functional... the proof is that your site still seems to work... at least for you... but what about on smartphones, what about updates... or what about updates in a year or two from now ?

so as @Nancy OShea recommends, you should consider migrating not only to a new, more contemporary work environment, but also to a major overhaul of your site, if it responds positively to the questions posed during the introduction of this comment.

in the meantime, if you wish to continue using CS5 from your new OS, consider using a virtual machine... https://www.virtualbox.org/  (it's totaly free) this way you can install your old operating system, then CS5...

depending on the version of the system you want to install, do a google on the subject... like "...install Lion on virtualbox..." and you find a step by step... (if not, come back here and ask us the question) Mac on Mac is not a problem, it's mostly Mac on Windows that is a legal problem

Nobody should be using outdated web authoring software in 2021.  The web has changed too much in 11 years.

================
MODERN CODE EDITORS:
-- Adobe Dreamweaver CC - https://www.adobe.com/products/dreamweaver.html
-- Atom (free) - https://atom.io/
-- Codespaces (free, browser-based) - https://github.com/features/codespaces
-- Nova (Mac only, formerly called Coda) - https://nova.app/
-- Pinegrow - https://pinegrow.com/
-- Sublime Text - http://www.sublimetext.com/
-- Visual Studio Code (free) - https://code.visualstudio.com/

==============
MODERN CODE TUTORIALS:

- https://www.w3schools.com/html/
- https://www.w3schools.com/css/
- https://www.w3schools.com/js/

Which OS do you have?

CS5 is legacy software from 2010.  The old installers won't run on Sierra, High Sierra or Mojave without terminal workarounds.

https://helpx.adobe.com/creative-cloud/kb/install-creative-suite-mac-os-sierra.html

32-bit CS5 won't run much install on 64-bit Catalina or Big Sur. It's too old.